Thermal inkjet (TIJ) technology is widely used in computer printers. TIJ cartridges typically comprise a print head mounted to a tape automated bonded circuit, commonly known as a "TAB circuit." TAB circuits are well known in the art as illustrated by U.S. Pat. Nos. 4,989,317 and 4,944,850. A print head typically has a plurality of precisely-formed nozzles, each nozzle being in fluid communication with a chamber that receives ink from an ink reservoir. Each chamber is adjacent to an electrical resistance element, known as a TIJ resistor, which is located opposite the nozzle so that ink can collect between the nozzle and the resistor. Electric printing pulses heat the TIJ resistor, causing a small portion of the ink adjacent to the resistor to vaporize, thereby propelling the ink through the nozzle of the print head toward a print medium. The ejected drops collect on the medium and form printed characters and/or images thereon. The printing is generally accomplished by incrementally moving the medium in a first direction relative to the print head and moving the print head in a second direction which is perpendicular to the first direction. As a plurality of nozzles may be supplied across the print head, a number of droplets may be fired from the print head simultaneously. The spacing of the nozzles and the incremental stepping of the print head and the medium define the resolution of the printed image. Further examples of this process can be found in U.S. Pat. No. 5,637,166.
Fabrication of a TIJ cartridge typically entails primarily three steps: the making of the print head itself; the making of a TAB circuit; and the alignment and attachment of the TAB circuit to the print head. The TAB circuit attaches to the print head so as to provide electrical contact and communication between the TIJ printer and the TIJ cartridge. As shown in FIG. 1, a TAB circuit 20 typically comprises a chip portion 30 and a contact pad portion 40. Electrical connection between contact pads 45 and the chip portion 30 is made by conductive traces 50. The electrical signals from the TIJ printer pass through the contact pad portion of the TAB circuit and travel to the chip portion of the TAB circuit. The chip portion of the TAB circuit is sometimes referred to as the heater chip. This chip will translate the signals passed to it from the contact pad portion of the TAB circuit to selectively heat small portions of adjacent ink. The ink is then propelled and collected onto a medium as described above.
Methods and apparatus for alignment and attachment of the TAB circuit to the print head are well known in the art and typically use a machine vision apparatus, for example as disclosed in U.S. Pat. No. 4,980,971, which is incorporated herein by reference.
Typically, the chip portion and the contact pad portion of the TAB circuit are attached on first and second intersecting surfaces, respectively, of the print head. In a conventional system, the chip portion of the TAB circuit is aligned by using a machine vision apparatus that optically locates two sets of datum points, one set on the first surface of the print head and one set on the TAB circuit, while a gripping arm holds the print head in place. Alignment of the chip circuit must be highly precise to ensure print quality and to accommodate ever increasing consumer demands for better print resolution.
Once alignment is achieved, the machine vision apparatus will then attach the chip portion of the TAB circuit to the first surface of the print head. Various methods for attachment are known in the art, as disclosed by U.S. Pat. No. 5,637,166 and can employ glues, seals, gaskets, heat staking, etc. Following alignment and attachment of the chip portion, a force is applied to the top surface of the chip portion of the TAB circuit and the first surface of the print head so as to maintain the chip alignment and attachment in the succeeding fabrication steps. Alignment may also be maintained with pressure sensitive adhesive or a heat activated bonding film. Additionally, alignment may be maintained by fast curing UV dots of adhesive placed under and around the chip portion of the TAB circuit.
Next, a gripper arm holding the contact pad portion of the TAB circuit rotates or folds the TAB circuit in order to align and attach the contact pad portion of the TAB circuit to the second surface of the print head using attachment methods as discussed above The attachment location on the second surface of the print head is determined by a computer-learned process. As a result, the TAB circuit is folded at or adjacent the intersection of the first and second surfaces of the print head.
The movement of the gripping arms and/or folding of the TAB circuit often apply forces to the already attached chip portion of the TAB circuit which result in misalignment of the attached chip portion. Additionally, the folding step can create a gap between the TAB circuit and the print head which is believed to be necessary in order to provide slack or movement in the contact pad portion of the TAB circuit during contact pad alignment and attachment. Yet, this gap is not a desirable feature. FIG. 2 illustrates a TAB circuit 20a, including a chip portion 30a and a contact pad portion 40a, which is aligned and attached to a print head 10a using a conventional technique. As shown in FIG. 2, a gap 160a often occurs where the TAB circuit is aligned at or adjacent the point of intersection 100a of surfaces 60a and 70a. Also, because the machine vision apparatus frequently needs operator intervention to reprogram and relearn contact pad alignment positions, frequent production delays and increased expense result. The equipment associated with the precision alignment of the contact pad portion to the second surface of the print head is also expensive. Finally, increasing demands in the market for the miniaturization of print cartridges have made it difficult to ensure accuracy of chip and contact pad alignment to a print head with the existing fabrication techniques in the art. Prior methods cannot provide the higher precision required with miniaturization because to do so requires smaller and more accurate drops of the gripping arms during contact pad alignment and attachment. These drops are already fraught with problems and to require them to be even more precise is not practical.
Accordingly, a need exists for methods and apparatus for improving this TAB circuit alignment on a print head.